The synthetic resin formed by the polymerization of propylene as the sole monomer is called polypropylene. While "polypropylene" has been used from time to time in the art to include a copolymer of propylene and a minor amount of another monomer, such as ethylene, the term is not so used herein.
The polypropylene of commerce is a normally solid, predominantly isotactic, semi-crystalline, thermoplastic polymer mixture formed by the polymerization of propylene by Ziegler-Natta catalysis. In such catalysis the catalyst is formed by an inorganic compound of a metal of Groups I-III of the Perodic Table, such as, an aluminum alkyl, and a compound of a transition metal of Groups IV-VIII of the Periodic Table, such as, a titanium halide). Typically the crystallinity of polypropylene thus produced is about 60% as measured by X-ray diffraction. As used herein, semi-crystalline means a crystallinity of at least about 5-10% as measured by X-ray diffraction. Also, the typical weight average molecular weight (Mw) of the normally solid polypropylene of commerce is 100,000-4,000,000, while the typical number average molecular weight (Mn) thereof is 40,000-100,000. Moreover, the melting point of the normally solid polypropylene of commerce is about 162.degree. C.
Although the polypropylene of commerce has many desirable and beneficial properties, it is deficient in melt strength or strain hardening (an increase in resistance to stretching during elongation of the molten material). Thus it has a variety of melt processing shortcomings, including the onset of edge weave during high speed extrusion coating of paper or other substrates, sheet sag and local thinning in melt thermoforming, and flow instabilities in co-extrusion of laminate structures. As a result, its use has been limited in such potential applications as, for example, extrusion coating, blow molding, profile extrusion, and thermoforming.
On the other hand, low density polyethylene made by a free radical process has desirable melt rheology for applications that require melt strength or strain hardening properties. Such low density polyethylene is believed to have these properties because the polymer molecules are non-linear. The molecules are chains of ethylene units that have branches of ethylene units of varying lengths. This non-linear structure occurs because of typical free radical inter- and intra-molecular transfer followed by further subsequent polymerization.
Low molecular weight, amorphous (predominantly atactic) polypropylene with a branched molecular structure is known in the prior art. See Fontana, Kidder and Herold, Ind. & Eng. Chem., 44 (7), 1688-1695 (1952), and the U.S. Pat. No. 2,525,787, to Fontana et al. It is disclosed as having been made by Friedel-Crafts catalysis. However, the molecular weight (weight average) of this polypropylene is at most about 20,000, the polymer is described as having normal (at 20.degree. C.) viscosity ranging from that of a light lubricating oil to that of a heavy oil or even resins of plastic or semi-solid nature, and its utility is reported to be as a blending agent and viscosity index improver for lubricating oils.
The crystalline polypropylene of commerce, however, is linear. That is, the polymer molecules are chains of propylene units without branches of propylene units. The reason is that in Ziegler-Natta catalysis secondary free radical reactions such as occur in the free radical polymerization of ethylene are highly improbable, if not non-existent.
Some effort has been made in the art to overcome the melt strength deficiency of the polypropylene of commerce.
One approach, as reflected in the U.S. Pat. No. 4,365,044, to Liu, and cited references thereof, has been to blend the linear polypropylene of commerce with a low density polyethylene which has the desirable melt strength or strain hardening properties alone or together with other polymeric substances. Although the blend approach has met with some success, it is not preferred.
Another approach to improve the melt properties of linear polypropylene is disclosed in the U.S. Pat. No. 3,349,018. According to this patent, linear polypropylene is degraded by subjecting it in air to ionizing radiation at a total dose from about 0.01 to about 3 megareps (equivalent to about 0.012 to about 3.6 megarads), but less than a dose at which gelation is caused. This patent discloses that radiation degraded linear polypropylene can be extruded and drawn at much higher linear speeds without the occurrence of draw resonance or surging. However, as can be determined from the patent, particularly Example VI, the neck-in of the in-air radiated linear polypropylene is actually greater than the neck-in of the non-irradiated linear polypropylene.
There are a number of references that disclose the ionizing radiation treatment of linear polypropylene. In the main, these references describe the resulting polymer either as degraded, as a result of chain scisson, or as cross-linked, as a result of polymer chain fragments linking together linear polymer chains. European patent application publication no. 190,889, published Aug. 13, 1986, describes high-molecularweight, long-chain branched polypropylene made by irradiating linear polypropylene with high-energy ionizing radiation. The free-end branched polymer is gel-free and has strain hardening elongational viscosity.
Likewise there are a number of references which disclose the peroxide treatment of linear polypropylene. Such references disclose either degradation or crosslinking of polypropylene by thermal or u.v. decomposition of the peroxides. Typically, degradation is the predominant reaction. Degradation or visbreaking of polypropylene by the thermal decomposition of peroxides is the common method used to narrow the molecular weight of the linear crystalline polypropylene. Generally, the resultant product consists of linear chains of polypropylene having both lower weight and number average molecular weights. Typically, the reaction is conducted at a temperature in excess of the melting point of polypropylene, i.e. in excess of 162.degree. C. However, when crosslinking is initiated by u.v. radiation, lower temperatures can be used. (See, Chodak, I. and Lazar, M., Effects of the Type of Radical Initiator on Crosslinking of Polypropylene, Die Angewandte Makromoledulare Chemie, 106, 153-160 (1982)) However, as is pointed out in this article, the lower temperatures decrease the decomposition rate of the peroxide initiator thereby leading to lower concentrations of the radical fragments of polypropylene, and decrease the mobility of these radical fragments of polypropylene thereby making recombination difficult. Although lower temperature is not defined in the article, the lowest temperature reported is in connection with crosslinking by u.v. irradiation of peroxides is 10.degree. C., with the optimal temperature for effective crosslinking being 65.degree.-80.degree. C.